T cell recognition of foreign antigens is mediated by the T cell receptor (TcR) and accessory CD8 or CD4 molecules. In the thymus, T cells differentiate from precursors which do not express TcR, CD8 or CD4 into mature TcR+ cells of CD8 and CD4 phenotype. T cell development progresses through a series of phenotypically separable steps. At some of these steps, immature T cells are subject to processes of positive and negative selection. The specificity of the TcR determines the fate of the thymocyte that bears it. the cells whose receptors can interact with self MHC molecules displayed by thymic cortical epithelial cells and self peptides bound to them are positively selected and allowed to complete maturation. Other thymocytes die without such positively selecting contact. Negative intrathymic selection eliminates all cells whose TcR possess exceedingly high affinity for self MHC and self peptides. It is not known at which stage(s) the selection processes operate, nor what are the molecular and biochemical correlates of these processes in the cell. Mature and immature T cells react differently to stimulation, and little is known of the signaling mechanisms in immature thymocytes. A long term goal of our experiments is to dissect these fundamental processes of T cell development. We will take advantage of methods used in our previous studies and the existence of TcR transgenic (Tg) mice which offer a simplified and magnified view into thymocyte biology. We will dissect phenotypic and functional stages of thymocyte development in TcR Tg mice, and correlate them to nontransgenic counterparts. We shall then manipulate thymocyte development using different micro-environments and various signaling agents which operate via TcR or other molecules, and examine the intracellular, phenotypic and functional consequences of these manipulations at different stages of maturation. We shall also overexpress MHC transgenic mice to study the efficacy of selection processes and the role of self MHC as a signal for immature thymocytes.